The invention relates to a multistage switching network which is useful in telephonic communication and, in particular, cellular mobile radio-telecommunication systems. Rearrangeable switching networks are known in the art, for example, U.S. Pat. Nos. 4,023,141; 3,978,291; 3,358,269; 3,129,407; 3,258,269; 4,038,638 and 4,022,982.
A rearrangeable full availability network is one in which a successful connection of any idle pair of terminals is guaranteed even though this may require the rearrangement of existing connections. While this same "guarantee" can be achieved in a "strictly nonblocking" network, such networks are larger and more expensive than networks with which the present invention is concerned.
The requirements of cellular mobile telecommunications place additional demands on the switching network employed in such systems. In a cellular system, a service area is typically divided into relatively small "cells," each cell having one or more radio antennae and a base station. All base stations are connected, via land lines to a Mobile Telecommunications Switching Office (MTSO) which serves as the control center for the system and which is also linked to the regular telephone network.
Mobile calls are initiated by the transmission of source and destination data over special "set-up" channels to the nearest base station which relays the signal to the MTSO. If the target unit (the number that was dialed) is a land-line telephone, the MTSO initiates a connection with the regular telephone network after assigning a pair of voice-channels to the mobile unit. If the target is mobile, the MTSO circulates a paging signal from one base station to the next until the target is located. The target responds by transmitting an acknowledgment to its local base station which is then relayed to the MTSO.
A mobile unit continues to use its assigned pair of channels for as long as it remains within its original (or current) cell. However, when a unit moves from one cell to another, a procedure, referred to as a "hand-off," is initiated. This procedure involves a further data exchange over the special set-up channel, the assignment of a new channel pair and the routing of the call to a new base station.
In normal telephone systems, once a connection is made it remains in effect until the conversation is terminated. On the other hand, due to hand-offs, the connecting paths in the cellular system are frequently broken and remade while the conversation is in progress. Clearly, therefore, the nature of the cellular system is such that it imposes a set of unique and stringent functional requirements on the switching network it employs. Specifically, the network must have full availability so that reconnections made during a hand-off can be guaranteed. For the same reason, the network must be highly reliable and have provisions for detecting and avoiding a faulty path and then detecting and isolating a faulty component for repair. Also, the reconnections or hand-offs should be transparent to the users, i.e., there should be no telltale discontinuities in ongoing conversations or loss of any data during hand-offs. Finally, reconnections must be made expeditiously and the design of the system must be such as to facilitate efficient control algorithms.
In the current state-of-the-art networks, all of the performance objectives associated with cellular radio-telecommunications cannot be achieved, or if achieved, the networks are relatively large and expensive and/or their architectures do not facilitate expeditious reconnections nor do they facilitate efficient control algorithms. Moreover, the state-of-the-art networks do not readily accommodate additions to their subscriber/user base. For example, in such a network having N.sub.T input/output terminals, additions to the network normally increase by a rate proportional to N.sub.T log.sub.2 (N.sub.T), rather than linearly with N.sub.T.
It is, therefore, an objective of the present invention to provide a rearrangeable fully available switching network useful for both voice and data transmission that meets all of the functional requirements of a cellular mobile radio-telecommunications system, but which can also be used in Private Automatic Branch Exchange (PABX) systems. Specifically, it is an objective of the invention:
(1) to provide a general multistage network configuration predicated on the use of redundant conductors between stages which, when compared to state-of-the-art networks, greatly reduces the probability of having to rearrange existing connections in order to connect an idle pair of terminals and which simplifies the process when rearrangement is required; PA0 (2) to provide a network configuration which facilitates path verification through the network and which facilitates the process of locating and isolating faulty components; PA0 (3) to provide conformable specific embodiments of the general network in which the least reducible element is a printed circuit board and in which all of the printed circuit boards are identical, throughout the network; PA0 (4) to provide conformable specific embodiments of the general network which permit the growth of the network, in terms of such printed circuit boards, to be directly proportional to the total number of input and output terminals required; PA0 (5) to provide conformable specific embodiments of the general network which permit the use of such printed circuit boards which are dual switching elements each of which consists of two disjoint elements with a common controller to effectively reduce the total number of such printed circuit boards by approximately one-half.